In combustion engines, particularly turbocharged natural gas engines, exhaust temperatures increase significantly near the upper end of an engine's operating range and/or at high altitudes. The high exhaust temperatures under these conditions are caused by the high exhaust manifold pressure required for such operations due to increased pumping work required of the turbocharger. The elevated exhaust manifold pressure causes slow combustion and reduces the margin for detonation, requiring retarded timing, thus reducing performance, efficiency, etc. Turbomachinery may often be detuned to maintain engine operation within the limits imposed by the elevated backpressure and elevated exhaust temperatures.
Also, engine speed in natural gas engines is often limited by exhaust temperature. Therefore it can be desirable to find ways to configure and operate natural gas engines in such a way as to produce lower exhaust temperatures in order to improve various aspects of engine performance. In turbocharged systems, the presence of the turbine in the exhaust flow path increases backpressure in the exhaust, which leads to higher exhaust temperatures. Exhaust temperatures may also be elevated due to high compression ratios, which may be used to improve performance. Thus, although the use of turbochargers and high compression ratios can improve performance, turbochargers and high compression ratios tend to increase exhaust temperatures to a degree that limits any potential performance improvements provided by turbochargers and high compression ratios.
In some cases, backpressure created by a turbocharger may elevate exhaust temperatures so significantly that the turbomachinery may be required to be detuned (i.e., not used to its full potential). For example, in order to improve low rpm power/torque output, a turbo that produces high levels of boost may be used. However, at higher rpm, the turbo may create such high backpressure, that the turbomachinery must be detuned by routing some of the exhaust around the turbine (e.g., with a wastegate) or by making adjustments to engine management (e.g., timing, fuel delivery, air/fuel mixture, etc.). While this may alleviate the backpressure issue at high rpm, this reduces the amount of exhaust driving the turbine and, therefore, reduces the boost produced by the turbo. Thus, the turbo cannot be used to its full potential.
In addition, in some applications, it may be desirable to be able to reduce engine speed, while maintaining torque output. For example, in the natural gas industry, the gas may be transported via a pipeline. In order to pump the gas through the pipeline, gas compressors may be used. These gas compressors may be driven by some sort of engine. Most commonly, a natural gas burning engine is used, because it may use the pipeline as a source of fuel.
It may be desirable, at certain times, to reduce the flow of gas through the pipeline. For example, when maintenance must be performed on part of the pipeline (e.g., a valve needs to be replaced), it may be desirable to slow, but not cease, the pumping of the gas. For example, when a valve downstream of the gas compressor needs replacement, a valve may be closed upstream of the valve needing replacement and the gas compressor may continue to run at a slower speed, so as to gradually build pressure between the gas compressor and the closed valve. When the service is complete, the closed valve may be opened, and the gas compressor may be brought back up to normal operating speed.
The easiest way to vary the speed of a compressor is to vary the speed of the engine driving it. Further, it has been determined that, because these compressors operate with relatively low frictional losses and low pumping losses, in many cases, the same amount of torque is required to drive these compressors regardless of the speed at which the compressors are run. Therefore, when it is desired to slow down the speed of an engine to reduce the speed of a compressor, it is desired that the torque output of the engine be maintained constant as the speed of the engine is reduced. This control strategy is commonly referred to as “turndown” or “speed turndown.”
It is also desirable to vary the speed of the compressor (and therefore the speed of the engine) over a wide operating range. This poses a challenge to produce a constant torque over a broad range of engine speeds.
In order to produce a constant torque over a broad range of engine speeds, systems have been developed that vary any of a number of engine operating parameters, such as, for example, turbo boost. Some systems vary turbo boost by using a turbo compressor bypass or an exhaust wastegate. As described above, however, the range of performance of a turbocharger can be limited by backpressure and exhaust temperature factors.
U.S. Pat. No. 6,343,473, issued to Kanesaka (the '473 patent), discloses an engine with an induction system including both a supercharger having a bypass, and a turbocharger. The engine in the '473 patent is disclosed as allegedly having the capability of producing a flat BMEP curve over a portion of its range of operating speeds.
The '473 patent, however, discloses that the engine is a diesel engine and not a natural gas engine. In addition, the '473 patent only discloses constant torque over less than half of its operating speeds.
Further, the system of the '473 patent is designed to purportedly solve problems that are not even posed by natural gas engines, particularly engines used for natural gas pipeline compression. For example, the supercharger disclosed in the '473 patent was purportedly added to reduce or eliminate turbo lag. However, for a steady state gas compression system, turbo lag is not an issue. Also, the '473 patent discloses the use of the supercharger in order to increase the amount of air delivered to the engine on startup, whereas natural gas engines tend to require intake throttling to reduce the intake charge delivered to the engine on startup. Furthermore, steady state gas compression systems are operated around the clock, 24 hours per day, so startup is a fairly infrequent occurrence. Accordingly, the '473 patent does not provide a solution for achieving broad ranges of speed turndown in natural gas engines, particularly those used for natural gas compression.
The present disclosure is directed to improvements in the ability of gas compression systems to undergo turndown, i.e., improvements in the range of turndown.